1. Field of the Invention
The present invention relates to an optical scan apparatus used for an image formation apparatus such as a digital copying machine or a laser printer as well as to an image formation apparatus incorporating the optical scan apparatus.
2. Description of Related Art
A prior art optical scan apparatus uses a polygon mirror or a galvano mirror for a deflector deflecting a light beam. In order to print images with high resolution at high speed, it is necessary for such an optical scan apparatus to heighten scan speed, that is, rotation speed of the deflector. However, there is a limitation to increasing the rotation speed of the deflector due to durability of bearings thereof, heat emission or noise caused by windage loss, or the like.
In view of the above problem, in recent years deflectors made by silicon micro-machining have been widely studied. For example, a deflector formed of a vibration mirror and a torsion shaft supporting the mirror integrally on a silicon substrate is disclosed in Japanese Patent Nos. 2924200, 3011144, 3445691, and 3543473 and Japanese Laid-open Patent Application Publication Nos. 2004-191416, 2005-308863, and 2004-279947.
Japanese Patent Nos. 2924200 and 3011144 disclose a deflector with a small-sized mirror surface which is reciprocatively vibrated by resonance, so that it generates less noise and consumes less power even in high-speed operation. Further, when used in an optical scan apparatus, such a deflector can contribute to a reduction in thickness of a housing of the apparatus due to its low vibration and almost no heat generation. Also, the housing can be made of low-cost resin materials containing less glass fibers without affecting quality of images.
Japanese Patent Nos. 3445691 and 3543473 disclose an optical scan apparatus with such a vibration mirror as a deflector instead of a polygon mirror. One problem of this vibration mirror is that a surface thereof may deform due to its own vibration. Japanese Laid-open Patent Application Publication No. 2004-191416 discloses a vibration mirror whose focus position is changeable in main scan direction in accordance with deflection angle thereof. Japanese Laid-open Patent Application Publication No. 2005-308863 discloses a vibration mirror whose shape is devised in such a manner as to suppress the deformation.
There is another problem in the vibration mirror that the deflection angle thereof varies depending on a change in spring constant of the torsion shaft due to temperature or a change in viscosity resistance of air owing to atmospheric pressure or the like. Japanese Laid-open Patent Application Publication No. 2004-279947 has dealt with this problem by configuring a vibration mirror that the deflection angle thereof is stably controlled by adjusting an applied current to the vibration mirror in accordance with deflection angle detected from a deflected light beam.
As described above, use of the vibration mirror not a polygon mirror as a deflector in an optical scan apparatus makes it possible to provide an image formation apparatus with less noise and less power consumption and adoptable for various ambient conditions and environments. Further, it is able to thin thickness of the housing of the optical scan apparatus owing to the low vibration of the vibration mirror, enabling weight saving and manufacturing cost saving therefor.
However, there still remains a problem that since the vibration mirror is extremely thin in thickness as several hundred μm, it is very difficult to form a flat mirror surface with high precision. When a light beam deflected by the mirror with insufficient precision illuminates an image plane, a spot size thereof differs according to a position (image height) of the imaging plane where it illuminates. A variation in the beam spot size causes deterioration in image quality, therefore, an image formation apparatus incorporating such an optical scan apparatus cannot form high-quality images.
It is possible to improve precision of the mirror surface by increasing thickness of a silicon substrate on which the vibration mirror is formed. However, the thickness increase leads to an increase in mass of the vibration mirror, resulting in reducing, at a same frequency, the deflection angle of the vibration mirror from that before the thickness increase.
Another problem is that due to its very thin thickness, undulating deformation occurs on the mirror surface. This is because force acts on the vicinity of a rotation axis and the ends of the vibration mirror in opposite directions because of a change in angular velocity of the mirror due to the vibration and inertia force on the mirror.
FIG. 22 schematically shows a prior art vibration mirror 23a with undulating deformation. Such deformation of the mirror surface leads to deterioration in wave aberration of a light beam reflected thereby, causing problems such as displacement of focus positions, deformation of beam profiles, or generation of sidelobes.
In view of solving the above problems, Japanese Laid-open Patent Application Publication No. 2004-191416 discloses an optical scan device which changes focus positions to correct displacement thereof which increases in accordance with the deflection angle of the light beam. However, the apparatus cannot still prevent displacement caused by the following factors when the light beam is incident on the mirror from directions outside a predetermined direction.
FIG. 23A shows a light beam incident on the vibration mirror 23a. In the drawing, incidence angle (between optical axis of the light beam and mirror surface) of the light beam decreases as the vibration mirror 23a rotates in a direction of arrows. Therefore, even with a same size of the light beam, the spot size of the light beam on the vibration mirror 23a increases in a direction perpendicular to the rotation axis (main scan direction) as it rotates in the direction of arrows.
Here, deformation of the mirror surface does not always occur symmetrically relative to the rotation axis. FIG. 23B shows a relation between deflection angle of the light beam and shape of the mirror surface of the vibration mirror 23a. As shown in the drawing, deformation of the mirror surface is small in the vicinity of the rotation axis (indicated by broken lines) so that the vibration mirror 23a has average optical power while the ends of the vibration mirror 23a is greatly deformed, being affected by inertia force so that it has greater optical power at the ends than in the vicinity of the rotation axis.
In other words, curvature of the mirror surface changes in the main scan direction depending on rotation angle of the vibration mirror 23a or a position thereon where the light beam illuminates. This causes the displacement of focus positions of the light beam, which further results in unevenness of spot size of the light beam on the surface of a photoconductive drum in the main scan direction and significant deterioration in image quality with nonuniform density or low resolution.
It is necessary to improve bending rigidity of the silicon substrate, that is, increase the thickness thereof on which the vibration mirror 23a is formed, in order to decrease the deformation of the mirror surface. However, again, the thickness increase causes the problem of mass increase of the vibration mirror 23a, resulting in reducing, at a same frequency, the deflection angle thereof from that before the thickness increase.